This invention relates to a method of forming a fluorescent screen for a cathode-ray tube, for instance, a color picture tube, and more particularly to a method of forming a fluorescent screen for a cathode-ray tube by coating a phosphor layer with an aqueous emulsion containing a water insoluble film forming resin, drying the film to form a base layer, depositing a metal film on the base layer and subjecting the base layer to pyrolysis to gasify and eliminate the same, in other words, evaporating the base layer.
The method of forming a metal coating on a fluorescent screen for a cathode-ray tube has been disclosed, for instance, in U.S. Pat. No. 3,067,055. Further, Japanese Patent Publications No. 24416/72 and No. 48263/72 propose a method wherein a water soluble film forming polymer is mixed with the aqueous emulsion which is a mixture of a water insoluble film forming resin and water and further, various additional substances are mixed with the resulting mixture in order to flatten the metal film and to reduce the blisters produced at the time of heating. The method comprises the steps of coating a phosphor layer with an aqueous emulsion of acrylate resin co-polymer, forming an evaporative base layer by heat-drying the film, depositing a metal film on the surface of the base layer and evaporating the base layer. The combination of the steps used for forming the base layer is termed "filming", the emulsion used for the purpose is termed "filming emulsion", and the step to evaporate the base layer is termed "heat-evaporation". The above method is characterized in that an emulsion of a mixture of complex boric acid of polyvinyl alcohol, hydrogen peroxide, potassium silicate and colloidal silica is added to the filming emulsion in order to prevent blisters on the metal film caused by the heat evaporation.
As the filming emulsion to be coated on the phosphor layer which has been formed on the inner surface of a cathode-ray tube bulb panel in the above conventional method, an aqueous emulsion has been used which is prepared by mixing a co-polymer of acrylic resin of a high pyrolysis temperature in the range of 400.degree. to 480.degree. C., complex boric acid of polyvinyl alcohol and water. The heat-evaporation step is generally carried out at the temperature in the range of 400.degree. to 450.degree. C. The prior art method has such drawbacks that when the evaporation step is carried out by frit-baking alone, omitting panel-baking, in order to shorten the time required for the heat evaporation, although organic substance such as the filming emulsion is partially gasified in the pyrolysis, some of the organic substance, not fully decomposed, is still left on the inner face of the panel. Also the gasified substance is adhered to the surface of a shadow mask mounted inside the tube of a completed bulb, deteriorating the shadow mask chemically. It has another drawback that the resin which is not decomposed remains on the fluorescent screen, lowering the luminescence characteristics of the cathode-ray tube, and reducing luminosity. When an aqueous emulsion of water insoluble resin is used in the filming step in the conventional method, the resin penetrates into the phosphor film to prevent the formation of a uniformly flat film and deteriorate the luminosity of the completed tube.
The uniformly flat film might be formed by coating the filming emulsion twice, but this makes the resinous film too thick to prevent blisters which might be caused on the metal film by gasified substance produced in the subsequent step, i.e. the heat-evaporation step, thereby causing such drawbacks as deteriorated luminosity or the defective film of the completed tube.